This invention relates to the measurement of pressure in fluids at relatively high temperatures.
It is frequently important to measure the pressure in a fluid that is in a region of relatively high temperature. Many types of apparatus are used to make such measurements, including manometers of various types, diaphragms, bellows and bourdon tubes. The foregoing elements are mechanical. Electrical pressure-measuring devices include piezoelectric crystals and devices which vary the capacitance of a capacitor as a function of pressure. All of the elements listed above have been used at various times and places to measure pressure. All suffer from various deficiencies when they are applied to measure the pressure in liquids that are at elevated temperatures and especially to liquid metals such as heated sodium and NaK (a mixture of sodium and potassium). Each of these liquid metals has been proposed for use in liquid-metal fast-breeder reactors. The possible presence of radioactivity in such reactors places additional requirements on the pressure sensors to be used.
Any sensor that is used to measure pressure in a liquid at a relatively high temperature must provide a reading that is either independent of the temperature or that can be corrected for temperature effects. Such a sensor must be reliable and it must be capable of calibration after it has been installed. A pressure sensor should provide an indication of pressure that is usable at a location remote from the measuring point and the delivery of information from the point of measurement to the point of use must not be disrupted by electrical noise or other interference.
One solution to the problem of measuring pressure in liquids is given in U.S. Pat. No. 4,103,555, entitled "Pressure Sensor for High-Temperature Liquids." Related solutions are given in U.S. Pat. No. 3,067,614, entitled "Apparatus for Indicating Pressure in Fluids System," and U.S. Pat. No. 2,883,995, "Pressure Transmitter for Liquid Metal System." The devices disclosed in all of these patents detect pressure by exposing a diaphragm to the system and measuring displacement of the diaphragm by some means. In the '555 patent the displacement effects a change in the capacitance of a capacitor, which change is measured by a capacitance bridge to provide a measure of the pressure. The '614 patent operates by detecting the variation of the inductance of an inductor as a core is caused to change its position in response to the movement of a diaphragm under pressure. The '995 patent restores the position of a diaphragm by applying an opposing pressure that is a function of the pressure to be measured.
Common to the systems described above is the use of a diaphragm or bellows that provides a pressure-measuring surface in contact with the fluid to be measured and a means of measuring motion of the diaphragm or bellows. In the '955 patent the availablity of a remote indication of pressure is a function of the passage of pressure lines to the remote location. The '614 and '555 patents make bridge measurements of displacement and the bridge outputs must be taken to remote locations for use. The pressure measurements, whether of electrical properties or a balancing pressure, will be analog in nature and hence will be more susceptible to disruption by electrical or other noise than would be a digital indication.
It is an object of the present invention to provide a better sensor of pressure in fluids at high temperatures.
It is a further object of the present invention to provide a digital pressure transducer for use in fluids at high temperatures.
Other objects will become apparent in the course of a detailed description of the invention.